It is very difficult to force someone to be a sex slave, they would have to have 24 hour guards posted and be watched 365 days a year, 24 hours per day. Have the threat of violence if they refused, and have no one notice and complain to the authorities or police. They would need to hide from the general public yet still manage to see customers from the general public and not have the customers turn the traffickers in to the police. They would need to provide them with medical care, food, shelter, and have all their basic needs met. They would need to have the sex slaves put on a fake front that they enjoyed what they were doing, act flirtatious and do their job well. They would have to deal with the authorities looking for the missing women, and hide any money they may make, since it comes from illegal activity. This is extremely difficult to do, which makes this activity rare.